Up to now, various fibers have been colored with various materials to impart a wide variety of color effect to the fibers. In recent years, particularly, attempts have been made to impart to fibers a high degree of value added by the use of various functional coloring materials as a coloring material. One of the products by such attempts is a fiber which contains a luminescent coloring material called a luminous pigment or a luminescent pigment.
JP-A-49-47646 and JP-B-3-70020 disclose luminescent textile products using, as the luminescent coloring material, a luminescent pigment comprising activated various metals with sulfides as main components. JP-A-2-112414 discloses a conjugated luminescent fiber comprising mainly zinc sulfide. However, these luminescence-emitting fibers disclosed as above exhibit only a low luminance and show a very short emission time of about 5 minutes. On the other hand, it is known that an addition of radioactive substances to such luminescent pigments improves their luminescent function, but the use of pigments containing radioactive substances in such daily used materials as fibers should be avoided in view of their adverse effect on human bodies.
JP-A-7-300722 discloses that a luminous pigment having a structure comprising metal oxides doped with rare earth elements can emit light for a long period of time with a high luminance although it substantially contains no radioactive substance. However, this fiber employs, as a thermoplastic resin used in a sheath component, hydrophilic polymers such as nylon-6, nylon-66, poly(ethylene terephthalate) and poly(butylene terephthalate), such that a luminous performance of the luminous pigment used is lowered due to a moisture contained in the resin.
In view of the above-mentioned problems, the object of the present invention is to provide a luminous fiber which can be used also for industrial materials, does not affect adversely on human bodies, and can emit light for a long period of time with a high luminance and in multiple colors, and a process for producing the fiber.